This invention relates to a projection exposure apparatus and a device manufacturing method. Particularly, the invention is suitably usable for the manufacture of large integration devices (semiconductor devices) of submicron or quarter-micron linewidth, such as ICs, LSIS, CCDs or liquid crystal panels, for example, through projection exposure wherein a pattern of a first object such as a mask or reticle is illuminated with light of a uniform illuminance distribution so that the pattern of the first object is transferred to a second object such as a silicon or glass wafer in accordance with a step-and-repeat method or step-and-scan method.
In projection exposure apparatuses for the manufacture of semiconductor devices, a reticle on which an electronic circuit pattern is formed is illuminated with light from an illumination system, and the pattern is projected and transferred to the surface of a wafer through a projection optical system. To provide improved resolution, it may be a factor for illuminating the wafer surface uniformly.
In illumination systems for such projection exposure apparatus, various measures may be adopted to assure illumination of a surface, to be illuminated (such as a reticle surface or wafer surface), uniformly. As an example, a projection exposure apparatus called a stepper may use an illumination system having a combination of an optical integrator, comprising small lenses arrayed two-dimensionally at a predetermined pitch, with a collimator lens, to illuminate the surface uniformly.
With the use of such an optical integrator in an illumination system, a plurality of secondary light sources corresponding to the number of small lenses are defined, and the surface to be illuminated is illuminated with lights from the secondary light sources, with these lights being superposed one upon another by means of the collimator lens. This is effective to provide a uniform illuminance distribution upon the surface to be illuminated, such as a mask surface or reticle surface.
Generally, non-uniformness of illuminance distribution on a surface to be illuminated, when the illuminance non-uniformness is S, the maximum of illuminance level upon the surface to be illuminated is Smax, and the minimum of it is Smin, is expressed as follows:
S=(Smaxxe2x88x92Smin)/(Smax+Smin).
Generally, in conventional projection exposure apparatuses, such non-uniformness of illuminance upon a surface to be illuminated is kept not greater than a few percent.
Another example is a method used in recent projection exposure apparatuses that distortion of a condenser lens is adjusted to correct a uniform illuminance distribution on a surface to be illuminated. Further, Japanese Laid-Open Patent Application, Laid-Open No. 42821/1989 proposes the use of light blocking members disposed at light entrance surfaces of some of small lenses, constituting an optical integrator, for blocking light impinging on these small lenses, to provide a uniform illuminance distribution on a surface to be illuminated.
The manufacture of semiconductor devices of large integration such as recent VLSIs, for example, requires very high uniformness of illuminance in circuit pattern printing.
It is known that, when the numerical aperture of a projection optical system of a projection exposure apparatus is NA and a wavelength used is xcex, the resolution RP and the depth of focus DOF can be expressed by the following equations:
RP=k1(xcex/NA)xe2x80x83xe2x80x83(1)
DOF=k2(xcex/NA2)xe2x80x83xe2x80x83(2)
where k1 and k2 are constants corresponding to a process, for example. In conventional illumination methods, when the numerical aperture of a projection optical system is NApo and the numerical aperture of an illumination optical system is NAil, the following parameter is an index of resolution:
"sgr"=NAil/NApoxe2x80x83xe2x80x83(3).
Conventionally, this "sgr"value is fixed to be about 0.5, and illuminance non-uniformness is reduced at this "sgr" value, whereby desired resolution is obtained.
However, in the manufacture of recent VLSIs, a further improvement of resolution is required for projection exposure apparatuses.
It is seen from equations (1) and (2) that, for an increase of resolution RP, xcex may be made smaller (shorter) and the numerical aperture NApo, of the projection optical system may be made larger. However, it leads to a decrease of depth of focus DOF.
For balancing of these contradictory factors, ultra-resolution imaging methods, called a grazing incidence illumination method or a phase shift mask method, have been proposed. In such an illumination method or phase shift method, an aperture stop of an illumination optical system is changed to make the "sgr" value smaller, or secondary light sources of a peculiar shape such as a ring-zone like shape or quadruple-pole shape, for example, are used.
For such illumination methods, in many projection exposure apparatuses, positions of various components of an illumination system are adjusted so that the illuminance non-uniformness becomes smallest in a certain standard illumination mode (illumination mode A). However. if the illumination mode is changed to another illumination mode (illumination mode B) of a small "sgr" value, based on the grazing incidence illumination method or a phase shift method, for example, sufficient reduction of illuminance non-uniformness is not always attainable with the same structure or disposition of the components of the illumination system.
Further, in a projection exposure apparatus, there are cases where flare occurs due to reflection among a wafer surface, a reticle surface, a projection optical system and an illumination optical system, which may cause a non-uniform illuminance distribution on the surface to be illuminated. The amount of such flare changes with the transmission factor of a reticle or a reflection factor of a wafer. It is difficult in conventional projection exposure apparatuses to correct a non-uniform illuminance distribution due to flare to provide a uniform illuminance distribution.
It is an object of the present invention to provide an improved projection exposure apparatus and/or device manufacturing method by which the illuminance distribution can be adjusted.
In accordance with an aspect of the present invention, there is provided a projection exposure apparatus, comprising: an illumination optical system for illuminating an original with light from a light source; and a projection optical system for projecting a pattern of the original, illuminated with the light onto a substrate to be exposed; wherein said illumination optical system includes an optical integrator having a plurality of lenses, and a movable member disposed at a light entrance side of said optical integrator and being movable in a direction intersecting an optical axis, said movable member having light quantity adjusting means for blocking a portion of light directed to a lens of said plurality of lenses to thereby change a light quantity distribution.
The direction intersecting the optical axis may be a direction perpendicular to the optical axis.
The movable member may be movable also in a direction of the optical axis.
The movable member may include a plurality of light quantity adjusting means each of which is provided in relation with an associated one of different lenses of said plurality of lenses.
In one preferred form of the present invention, the position of the movable member with respect to the direction perpendicular to the optical axis is changed to correct asymmetry of an illuminance distribution, with respect to the optical axis, upon a mask, a reticle or a wafer. The position of the movable member with respect to the optical axis direction may be changed to correct an illuminance difference, upon a mask, a reticle or a wafer, between a region close to the optical axis and a region remote from the optical axis. These corrections may be performed on the basis of a measurement of an illuminance distribution on the mask, the reticle or the wafer. The measurement and correction may be made once or plural times until a desired illuminance distribution is provided. Once the position of the movable member with respect to the optical axis direction and the position thereof with respect to the direction perpendicular to the optical axis, with which a desired illuminance distribution can be provided on the mask, the reticle or the wafer, are determined, the position of the movable member with respect to the optical axis direction and the position thereof with respect to the direction perpendicular to the optical axis may conveniently be memorized into memory means.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.